(Ph) 5. Geomechanics
Pillar design at Phalen Colliery was based on sea bed strain and overlap between walls. Pillar design was later checked using the Wilson's method and ALPS (Analysis of Longwall Pillar Stability).
The geotechnical properties of coal and rockmass at Phalen Colliery are provided by the Cape Breton Development Corporation as follows:
Rock type
UCS
Tensile Strength
Young's Modulus
Poisson's Ratio
Mean
Std.Dev.
Mean
Std.Dev.
Mean
Std.Dev.
Mean
Std.Dev.
1. Conglomerate
88.52
13.02
-
-
26.92
5.08
0.32
0.05
2. Medium grained sandstone
75.33
22.36
5.10
1.57
19.39
4.79
0.30
0.07
3. Fine grained sandstone
96.80
15.32
6.20
1.34
21.05
4.08
0.33
0.06
4. Combined sandstone
90.56
20.18
5.57
1.57
20.63
4.33
0.32
0.06
5. Laminate
74.20
22.19
6.90
1.06
17.79
6.17
0.26
0.11
6. Siltstone
82.55
33.30
6.76
2.77
21.86
5.06
0.30
0.07
7. Mudstone / Shale
32.56
14.45
32.56
14.45
13.87
5.30
0.31
0.17
8. Limestone
43.87
5.46
43.87
5.46
15.24
3.37
0.20
0.05
The roof strata at Phalen Colliery were composed primarily of sandstone, siltstone, and Mudstone. Dr. C. Mark attributed a CMRR rating of 35 to the #8 East Panel. Studies carried out on nine boreholes in #7 East Panel revealed the following data, which are also shown in Figure Ph6.
Borehole
Borehole
Borehole
Immediate Roof
Sandstone Main Roof
Number
Position(1)
Length (m)
h-imm (m)(2)
RQD(3)
h-total (m)
RQD
h-1(4) (m)
PH-155
144
20.98
7.72
25
6.56
31
6.56
156
596
25.30
4.60
24
10.0
60
3.63
157
843
28.65
5.13
08
21.23
67
5.69
108
377
25.30
8.29
08
13.11
14
13.11
107
879
22.86
3.71
0
17.52
08
5.29
205
1311
24.63
7.25
22
11.65
39
7.25
109
1743
28.65
6.10
-
19.10
-
17.0
202
2286
27.09
3.46
18
23.63
18
8.9
PH-201
3006
25.03
5.30
22
16.70
62
4.5
(1): Borehole position is the distance from the starting faceline of #7 East Panel.
(2): h-imm stands for the immediate roof thickness.
(3): Average RQD from borehole log sheets.
(4): h-1 is the estimated thickness of the lower layer of sandstone separated by local shards / clasts / thin beds.
In the immediate roof of the Phalen Seam, the Flood Plain and Backswamp Facies were highly susceptible to bed separation. The unsecured stand-up time of these facies was less than an hour. Shales and mudstones within the immediate roof displayed stand-up times that could be measured in the order of minutes. Figure Ph7 illustrates the interpreted roof strata along part of the #7 East Top and #7 East Bottom.
Tectonically, the Sydney Coalfield had not been appreciably disturbed but remnants of this history has been preserved in the form of elevated horizontal stress. The direction of principal horizontal stress was found by measurement (overcoring) to be North-East South-West (at 240º azimuth according to Dr. S. Zou) and about twice the vertical stress in magnitude. The stress was well aligned with field observations.
The high horizontal stress concentration has been used to account for the severe 'cutter roof' that is usually found along one of the ribs in Phalen roadways. The thin, intercalated sandstone beds frequently exhibited core disking due to the high stress. The core disking phenomenon was observed in several boreholes in #7 East Panel (Figure Ph7):
The horizontal stress induced bed separation in laminated rockmass. Roof conditions varied depending upon the existence or absence of the said sandstone beds.